This invention relates generally to multiplexing data into a message and transmitting the message within a data transmission network.
The Synchronous Optical Network (SONET) is a data communications standard and protocol that standardizes the rates and formats of different types of data communicating between different networks. SONET allows data of different formats such as DS1, DS2, and video to be combined together in a common data message format that can be multiplexed with other messages and transmitted together over a single fiber optic line. The SONET standard allows the multiplexing of these lower rate signals directly into a higher speed data stream without having intermediate stages of multiplexing.
As illustrated in FIG. 1, the basic SONET message 100 known as an STS-1 has a transmission time of 125 μ-seconds and includes 9 rows by 90 columns of data, divided into two parts. The first part is the transport overhead data 102, which includes the data in columns 1-3. The transport overhead includes a section overhead portion and a line overhead portion. The section overhead includes framing data that is used to determine if an out-of-frame error occurs. The line overhead portion includes parity check bytes that are the results of a parity calculation performed on the message.
The second part of the STS-1 message is the payload data 104, which includes the data in columns 4-90. An additional one column of overhead (not shown), referred to as the path overhead, is also provided in the first column of the payload data 104. Thus each SONET message has only 4.44% of overhead in any message. The SONET standard requires that no additional overhead data is added to the SONET messages as they are multiplexed together
N SONET STS-1 messages are multiplexed together into an STS-N message using byte interleaving, such that each byte of each individual STS-1 message is visible within the multiplexed STS-N message. In this way the entire message does not need to be de-multiplexed in order to access an individual STS-1 message. However, when an STS-1 message is added or removed from the multiplexed STS-N message, new line and section overhead bytes must be regenerated since some message parameters such as framing and parity may have changed during the multiplexing operations. Because of this, error data contained in the transport overhead bytes may be lost during this process. Thus, data relating to the bit errors that occur as the message passes through the SONET network, or data relating to any out-of-frame error conditions that occur during the transmission of data through the SONET network will not necessarily be available to the client when they receive the message at the egress of the network.
Therefore it would be desirable to provide the capability to preserve some predetermined error data during the transmission of the client data through a SONET network.